Semiconductor components based on silicon carbide as base material are continuously developed to be used in connection with high temperatures, high power applications and under high radiation conditions. Under such circumstances conventional semiconductors do not work satisfactorily. Evaluations indicate that SiC semiconductors of power MOSFET-type and diode rectifiers based on SiC would be able to operate over a greater voltage and temperature interval, e.g. up to 650-800.degree. C., and show better switching properties such as lower losses and higher working frequencies and nevertheless have a volume 20 times smaller than corresponding silicon components. These possible improvements are based on the favourable material properties that silicon carbide possesses in relation to silicon, such e.g. a higher breakdown field (up to 10 times higher than silicon), a higher thermal conductivity (more than 3 times higher than silicon) and a higher energy band gap (2.9 eV for 6H-SiC, one of the crystal structures of siC).
As SiC semiconductor technology is relatively young and in many aspects immature, there are many critical manufacturing problems that are to be solved until SiC semiconductor devices may be realized experimentally and manufacturing in a large number may become a reality. This is especially true of components intended for use in high-power and high-voltage applications.
One of the difficulties to overcome when manufacturing high voltage diodes or other types of semiconductor components comprising a voltage absorbing pn junction is to produce a proper junction termination at the edge of the junction. The electric field at the periphery of the junction is normally enhanced compared to the electric field in the bulk of the junction. This field increase at the periphery of the junction may be further reinforced in the presence of surface charge.
A high electric field at the edge of the pn junction implies a great risk of voltage breakdown or flash-over at the edge of the junction as well as giving rise to an instability of the blocking voltage known as voltage drift.
To avoid said disadvantages it becomes very important to reduce the field concentration, where the junction reaches the surface. Combined with efforts to passivate the surface of the component, measures are taken to flatten out the electric field at the surface e.g. by acting on how the pn junction emerges at the surface. As an example it is known from silicon power components to lap (grind, sandblast, etch) the surface of the edge to a certain angle in relation to the pn junction to thereby flatten out the field. Another known technique is to gradually decrease the charge content on the highly doped side of the junction, such that the charge content of the highly doped layer is reduced towards the outermost edge of the junction (so called Junction Termination Extension, JTE). The methods, known from silicon technology, used to achieve a JTE of an Si component are difficult or almost impossible to apply to components based on silicon carbide due to the great hardness of the material and extremely low diffusivity of proper SiC dopants. As an example, doping through diffusion is not feasible for SiC, as diffusion coefficients are negligible below 2270.degree. K. Also, ion implantation of doping elements, a common technique when manufacturing Si components, is difficult to master and not yet fully developed for SiC. Hence, many of the problems reminiscent of those prevalent at the beginning of the development of corresponding silicon components to be solved when developing semiconductor components from SiC have not been solved as yet for pn junctions in SiC.
High voltage diodes from 6H-SiC with epitaxially formed pn and Schottky junctions have been made experimentally (see e.g. M. Bhatnagar and B. J. Baliga, IEEE Trans. Electron Devices, vol. 40, no. 3 pp 645-655, March 1993 or P. G. Neudeck, D. J. Larkin, J. A. Powell, L. G. Matus and C. S. Salupo, Appl. Phys. Lett. vol 64, No 11, Mar. 14, 1994, pp 1386-1388). Some of the problems related to SiC devices have thus been solved, but no reliable solution to the problems connected with electric field concentration at the edges of the junction has been presented as yet.
The electric field may be reduced at the edge of the pn junction by applying a semi-isolating layer to the edge of the junction of an SiC component. Such a solution is described in document PCT/SE94/00482.
Any method or device to accomplish a semiconductor component corresponding to the principle of Junction Termination Extension at a pn junction composed of Si is not publicly known for a component, where SiC constitutes the base material of the junction. Solutions for arriving at SiC components comprising pn junctions with JTEs are described in the unpublished patent application U.S. Ser. No. 08/520,689, which is hereby included in this description by reference. The solutions described there involve stepwise decreasing charges of the JTE towards the edge of the JTE by use of an etch down technique, epitaxial regrowth or ion implantation in order to control the surface doping and surface fields. The present invention aims at describing a voltage absorbing edge at a pn junction with a JTE structure of an SiC component where the pn junction has a planar structure.
The term SiC is used in the following text to refer to any of the principal crystal polytypes of this material known as 6H, 4H, 2H, 3C and 15R.